The present invention relates generally to chemical mechanical polishing of substrates, and more particularly to a carrier head for a chemical mechanical polishing system.
Integrated circuits are typically formed on substrates, particularly silicon wafers, by the sequential deposition of conductive, semiconductive or insulative layers. After each layer is deposited, the layer is etched to create circuitry features. As a series of layers are sequentially deposited and etched, the outer or uppermost surface of the substrate, i.e., the exposed surface of the substrate, becomes increasingly non-planar. This non-planar outer surface presents a problem for the integrated circuit manufacturer. If the outer surface of the substrate is non-planar, then a photoresist layer placed thereon is also non-planar. A photoresist layer is typically patterned by a photolithographic apparatus that focuses a light image onto the photoresist. If the outer surface of the substrate is sufficiently non-planar, then the maximum height difference between the peaks and valleys of the outer surface may exceed the depth of focus of the imaging apparatus, and it will be impossible to properly focus the light image onto the outer substrate surface.
It may be prohibitively expensive to design new photolithographic devices having an improved depth of focus. In addition, as the feature size used in integrated circuits becomes smaller, shorter wavelengths of light must be used, resulting in a further reduction of the available depth of focus. Therefore, there is a need to periodically planarize the substrate surface to provide a substantially planar layer surface.
Chemical mechanical polishing (CMP) is one accepted method of planarization. This planarization method typically requires that the substrate be mounted to a carrier or polishing head. The exposed surface of the substrate is then placed against a rotating polishing pad. The carrier provides a controllable load, i.e., pressure, on the substrate to press it against the polishing pad. In addition, the carrier may rotate to provide additional motion between the substrate and polishing pad. A polishing slurry, including an abrasive and at least one chemically-reactive agent, may be distributed over the polishing pad to provide an abrasive chemical solution at the interface between the pad and substrate.
A CMP process is fairly complex, and differs from simple wet sanding. In a CMP process, the reactive agent in the slurry reacts with the outer surface of the substrate to form reactive sites. The interaction of the polishing pad and the abrasive particles with the reactive sites results in polishing.
An effective CMP process should have a high polishing rate and generate a substrate surface that is finished (lacks small-scale roughness) and flat (lacks large-scale topography). The polishing rate, finish and flatness are determined by the pad and slurry combination, the relative speed between the substrate and pad, and the force pressing the substrate against the pad. Because inadequate flatness and finish can create defective substrates, the selection of a polishing pad and slurry combination is usually dictated by the required finish and flatness. Given these constraints, the polishing rate sets the maximum throughput of the polishing apparatus.
The polishing rate depends upon the force with which the substrate is pressed against the pad. Specifically, the greater this force, the higher the polishing rate. If the carrier head applies a non-uniform load, i.e., if the carrier head applies more force to one region of the substrate than to another, then the high pressure regions will be polished faster than the low pressure regions. Therefore, a non-uniform load may result in non-uniform polishing of the substrate.
One problem that has been encountered in CMP is that the edge of the substrate is often polished at a different rate (usually faster, but occationally slower) than the center of the substrate. This problem, termed the xe2x80x9cedge effectxe2x80x9d, may occur even if the load is uniformly applied to the substrate. The edge effect typically occurs in the perimeter portion, e.g., the outermost five to ten millimeters, of the substrate. The edge effect reduces the overall flatness of the substrate, makes the perimeter portion of the substrate unsuitable for use in integrated circuits, and decreases yied.
Therefore, there is a need for a CMP apparatus that optimizes polishing throughput while providing the desired flatness and finish. Specifically, the CMP apparatus should have a carrier head which provides substantially uniform polishing of a substrate.
In one aspect, the invention is directed to a carrier head for use in a chemical mechanical polishing system. The carrier head comprises a base and a flexible member connected to the base to define a first chamber, a second chamber and a third chamber. A lower surface of the flexible member provides a substrate receiving surface with an inner portion associated with the first chamber, a substantially annular middle portion surrounding the inner portion and associated with the second chamber, and a substantially annular outer portion surrounding the middle portion and associated with the third chamber. Pressures on the inner, middle and outer portions of the flexible member are independently controllable.
Implementations of the invention may include the following. The width of the outer portion may be significantly less than the width of the middle portion. The outer portion may have an outer radius approximately equal to or greater than 100 mm, such as 150 mm, and the width of the outer portion may be between about 4 and 20 mm, such as 10 mm. The flexible member may include an inner annular flap, a middle annular flap, and an outer annular flap, each flap being secured to a lower surface of the base to define the first, second and third chambers.
In another aspect, the carrier head comprises a flange attachable to a drive shaft, a base, a gimbal pivotally connecting the flange to the base, and a flexible member connected to the base and defining a chamber. A lower surface of the flexible member provides a substrate receiving surface. The gimbal includes an inner race connected to the base, an outer race connected to the flange to define a gap therebetween, and a plurality of bearings located in the gap.
Implementations of the invention may include the following. A spring may urge the inner race and outer race into contact with the bearings, and an annular retainer may hold the bearings. A plurality of pins may extends vertically through a passage in the flange portion such that an upper end of each pin is received in a recess in the drive shaft and a lower end of each pin is received in a recess in the base portion to transfer torque from the drive shaft to the base. A retaining ring may be connected to the base to define, in conjunction with the substrate receiving surface, a substrate receiving recess.
In another aspect, the invention is directed to an assembly for use in a chemical mechanical polishing system. The assembly comprises drive shaft, a coupling slidably connected to the drive shaft, a carrier head secured to a lower end of the drive shaft to rotate with the drive shaft, a vertical actuator coupled to an upper end of the drive shaft to control the vertical position of the drive shaft and the carrier head, and a motor coupled to the coupling to rotate the coupling to transfer torque to the drive shaft.
Implementations of the invention may include the following. The drive shaft may extend through a drive shaft housing, and the vertical actuator and the motor may be secured to the drive shaft housing. The coupling may include an upper rotary ring surrounding the upper end of the drive shaft and a lower rotary ring surrounding the lower end of the drive shaft, a first bearing rotatably connecting the upper rotary ring to the drive shaft housing and a second bearing rotatably connecting the lower rotary ring to the drive shaft housing. The upper and lower rotary rings may be spline nuts and the drive shaft may be a spline shaft.
In another aspect, the invention is directed to a carrier head assembly for use in a chemical mechanical polishing system, comprising a drive shaft a first ball bearing assembly laterally securing an upper end of the drive shaft, a second ball bearing assembly laterally securing a lower end of the drive shaft, and a carrier head connected to the lower end of the drive shaft by a gimbal. The gimbal permits the carrier head to pivot with respect to the drive shaft. The distance between the first ball bearing assembly and the second ball bearing assembly is sufficient to substantially prevent lateral forces transferred through the gimbal from pivoting the drive shaft.
In another aspect, the carrier head assembly comprises a drive shaft and a carrier head connected to a lower end of the drive shaft. The drive shaft includes a bore and at least one cylindrical tube positioned in the bore to define a central passageway and at least one annular passageway surrounding the central passageway. The carrier head includes a plurality of chambers, each chamber connected to one of the passageways.
Implementations of the invention may include the following. The draft shaft may include two concentric tubes positioned in the bore to define three concentric passageways, each of the passageways connected to one of the chambers. A rotary union may couple a plurality of pressure sources to respective ones of the plurality passageways.
In another aspect, the invention is directed to a carrier head comprising first, second and third independently pressurizable chambers, a flexible inner member associated with the first chamber to apply a first pressure to a central portion of a substrate, a substantially annular flexible middle member associated with the second chamber and surrounding the inner member to apply a second pressure to a middle portion of the substrate, and a substantially annular flexible outer member associated with the third chamber and surrounding the middle member to apply a third pressure to an outer portion of the substrate. The outer member is substantially narrower than the middle member.
Advantages of the invention include the following. The carrier head applies a controllable load to different portions of the substrate to improve polishing uniformly. The carrier head is able to vacuum-chuck the substrate to lift it off the polishing pad. The carrier head contains few moving parts, and it is small and easy to service.
Other advantages and features of the present invention will become apparent from the following description, including the drawings and claims.